First, WindowsMaker doesn't use Objective-C, it's written in C. However, GNUstep, which is the open source implementation of the Cocoa frameworks (originally the OpenStep specification, but they're tracking Apple changes) could use more help! Oh, and we support (on *NIX) a superset of the Objective-C language that Apple supports on their products, so I wouldn't say that Obj-C is more limited on Linux.
That said, and I say this as the maintainer of the GNUstep Objective-C implementation, I'd recommend C++, but with the two caveats:
- C++ is not an OO language. It sort-of supports OOP, but writing OO code in C++ is not the natural way of using the language.
- Don't look at any version of the language before C++11. It's just terrible and will damage your brain.
C++11 and C++14 have cleaned up C++ a lot. With shared_ptr and unique_ptr, you can write code with sane memory management. With perfect forwarding, lambdas, and variadic templates, you can write code that has most of the benefits of a late-bound language. I like a lot of Objective-C, but Apple broke the 'simple, orthogonal syntax' when they added declared properties and a few other things. Any successful programming language eventually becomes a mess of compromises and ugly corners. Some, like Python and C++, start that way, but at least C++ has been slowly improving over the last couple of versions.
The one thing where Objective-C is still a clear winner is in writing libraries that want to maintain a stable ABI. This is insanely difficult in C++ because the language doesn't have a clean separation of interface and implementation and relies a lot on inlining and static binding for performance. The down side, of course, is that once you have a library in Objective-C you're limited to consumers who also want to use Objective-C.
Oh, and Qt GUIs suck beyond belief on OS X - not sure what they're like on Windows, but I wouldn't recommend them for a portable UI. Good MVC design and a native UI is the only way to go if you really want a cross-platform GUI app that doesn't suck.